1. Field of the Invention
This invention relates to inkjet printers and, in particular, to a printing technique for reducing the dry time of ink or increasing the adherence of ink printed on a medium.
2. Background
U.S. patent application Ser. No. 09/273,703, entitled "Mixing Overprinting and Underprinting of Inks in an Inkjet Printer to Speed up the Dry Time of Black Ink Without Undesirable Hue Shifts," by Paul Gast et al. (including Brooke Smith as a co-inventor), was filed on Mar. 22, 1999. This prior application is incorporated herein by reference. This prior application describes underprinting a pigment-based black ink with a dye-based color ink for reasons including speeding up the dry time of the black ink. Overprinting the black ink with a different color ink was also described for avoiding hue shifts in bi-directional printing. Fixed amounts of the overprinted and underprinted inks were described. This technique works well; however, the present Applicants have discovered that, at a cost of additional complexity, the technique can be improved by dynamically selecting the amount of ink to be deposited for the overprinting and/or underprinting. This improvement is the subject matter of the present application.
A typical high quality color inkjet printer prints using at least four colors of ink: cyan, magenta, yellow, and black. A common black ink is a pigment-based ink where undissolved particles are suspended in a clear vehicle. Such pigment-based ink creates the darkest black with a minimum of bleed into the paper. Since the paper is typically white, any significant bleeding of the black ink into the paper will noticeably reduce the sharpness of the edges of black text or other black graphics.
For non-black color inks, dye-based inks are very popular. Dye-based inks do not have color particles suspended in solution and thus tend to bleed into the paper more than pigment-based inks. Since the dye-based ink wicks or bleeds into the paper, the dye-based inks dry faster than the pigment-based inks, which effectively pool on the paper surface. Non-black color inks may also be pigment-based.
Examples of such inks are described in U.S. Pat. Nos. 5,695,820 and 5,626,655 assigned to the present assignee and incorporated herein by reference.
Fixers are well known clear solutions that are sometimes printed under pigment-based, dye-based, or pigment/dye-based hybrid inks to help the inks bond to the paper. Fixers may also be used to minimize finger smudging of the ink, as well as for other benefits. One type of fixer is a cationic polymer. Fixers are described in U.S. Pat. Nos. 4,694,302 and 5,746,818, incorporated herein by reference. Such fixers are printed by a print cartridge (or pen) similar to those used to print the black and other color inks. Fixers are optional in the present invention.
As inkjet printers evolve to print faster, there is less time for the ink to dry. In some cases, after printing on a page is complete, the printer needs to hold onto the page for a predetermined time in order to let the ink dry before depositing the page in an output tray. The drying of the pigment-based black ink is likely to be the bottleneck for drying time of a page.
The prior application, describing using a fixed volume of underprinting ink, solves the problem of drying time of the black ink by first wetting the paper with a dye-based ink followed by printing with the black ink over the wetted paper. The paper wicks the clear vehicle for the black pigment to hasten the black ink's drying time. In this example, such underprinted fluid acts as a surfactant. In another embodiment, the fixer or dye-based ink used to underprint a pigment-based ink reacts with the black ink to precipitate out the black pigment and quickly solidify the pigment. These underprinted fluids are referred to as reactants. Reactants also serve to prevent bleeding of the black ink into any adjacent color ink patterns. The underprinted fluid can have characteristics of both reactants and surfactants.
There is an optimum volumetric relationship between the "image" ink and the underprinting (or overprinting) ink or fixer. It is difficult to provide a fixed amount of ink per ink drop since the drop weight varies as a function of pen temperature, pen firing frequency, and pen life. Thus, ideally, the amount of underprinting (or overprinting) fluid must track the changing drop weight of the image ink. Further, the optimum underprinting (or overprinting) level is affected by the media characteristics, which are a function of ambient air temperature and relative humidity.
What is needed is a technique that provides a more optimum amount of underprinting (or overprinting) fluid as characteristics within the printer change.